In a typical AR-pattern firearm, in which a recoil spring is located in either the butt stock or a tubular extension of the receiver aft of a bolt carrier assembly, a recoil buffer may serve multiple purposes. In general, the buffer is a reciprocating mass that provides an interface between the recoil spring and the rear end of the reciprocating bolt carrier. The buffer and recoil spring typically remain captive within the butt stock or a tubular extension of the receiver so that an upper receiver, that houses the bolt carrier, and a lower receiver, to which the butt stock or extension tube is attached, are easily separated and reassembled.
A head portion of the buffer provides a forward contact surface for contact with the rear end of the bolt carrier and a sliding surface that guides the buffer within the extension tube. The body of the buffer that is orientated rearwardly relative to a head portion is received within the helical recoil spring. The tubular body is generally provided in two standard lengths: rifle and carbine. The aft end of the buffer typically includes a bumper made of at least somewhat resilient material to absorb some impact when the recoil spring has been compressed to the length of the buffer body and the reciprocating mass (buffer and bolt carrier assembly) reaches the end of its recoil stroke.
It is possible to “tune” how the firearm cycles by adjusting the weight of the reciprocating mass. This is most easily accomplished by choosing a buffer having a selected weight. Typically, the weight of a standard buff is from about 3 ounces for a carbine to 8 ounces or more for specialized rifle buffers. Increasing the buffer weight of the reciprocating mass can provide noticeable reduction in felt recoil and muzzle movement. Providing additional mass can also aid feeding of ammunition cartridges during the loading cycle. Specialized buffer designs having moving parts or pistons have also been used.
Traditionally, a standard recoil buffer has been constructed from four component parts: a tubular body having a closed forward end and an opposite open end, a plug used to close the open end and to provide a bumper at the aft end, a cross pin used to secure the closure plug to the body, and an internal weight or weights. The tubular body is typically machined from metal stock (such as an aluminum alloy). The cross pin is typically a roll pin that extends through a transverse opening in the plug that is aligned with opposite openings in the tubular sidewalls of the body adjacent the open end. The internal weight is typically three or more solid pieces sized to fit within the tubular body, each being made from steal or tungsten so that a combination of pieces, depending on material, will provide a selected total weight for the buffer. Typically, the body, plug, and roll pin may make up about 1.2 ounces of the total weight with a combination of steel and/or tungsten inserts providing the balance of the total weight. The traditional buffer is assembled by providing the metal tubular body, inserting a selected combination of weights, inserting the plug/bumper, and then hammering a roll pin into place to secure the plug. Alternative constructions have used a threaded engagement between the plug and body. Each of these is a removable engagement that may be disassembled but, as a result, may also come apart when unintended.